The increasing use of sulfur-containing fuels as energy sources, namely coal and oil, has resulted in pollution of the atmosphere throughout the world. The burning of the fuel produces flue gases containing substantial quantities of SO.sub.x, predominantly SO.sub.2 and SO.sub.3.
A wide variety of processes have been proposed for the removal of the SO.sub.2 from flue gases before they are emitted to the atmosphere. Processes using dry solids, liquids or slurries of sorbents injected in a manner to directly contact the flue gases have proven to be effective for control of SO.sub.2 emissions. Of these, the calcium-based wet scrubber systems have experienced the greatest use in industrial and utility application. This involves finely grinding lime or limestone of suitable composition, forming a water slurry or mixture, and providing apparatus and method for slurry contact with the flue gases. The SO.sub.2 reacts with the calcium reagent to form calcium sulfate/sulfite which is collected and removed in the form of a generally thixotropic sludge. This sludge is usually difficult to dispose because it does not easily dewater, and heavy metals can leach from the waste sludge containment ponds.
Further, abrasion to the slurry pumps and other scrubber handling equipment (due to the hardness of the limestone particles) increases maintenance costs. The operating costs are increased because the evaporation of water in the system cools the flue gases nominally by 200.degree. F. The moist flue gases, containing as they do some residual dissolved SO.sub.x, are also acidic. In addition, where the utility or industrial plant is located in an arid region, the water requirements of a wet pollution control process are expensive and drain an already scarce natural resource.
The subject of flue gas desulfurization by the injection of dry sodium-based reagents has been extensively investigated in laboratory, pilot, and full scale applications since the 1960's. These investigations have provided the technical basis to evaluate the economic aspects of the technology and compare its advantage to the current calcium-based technologies. The injection of dry reagents clearly is the least demanding of capital funds for both new and retrofit applications. The use of familiar hardware such as pulverizers, blowers, and silos permit the easy installation and operation of this simple technology.
Accordingly, the injection of dry reagents into flue gases have become of increasing interest. Principal among these is the use of sodium compounds as a reagent or sorbent. Sodium-based reagents that have been used are commercial sodium bicarbonate (NaHCO.sub.3), light and dense soda ash (Na.sub.2 CO.sub.3), sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O), trona (a naturally occurring form of sodium sesquicarbonate), and Nahcolite (a naturally occurring form of sodium bicarbonate). The reagents of greatest effectiveness are commercial sodium bicarbonate and Nahcolite. The sodium bicarbonate or Nahcolite is injected into the flue gas duct of a utility or industrial plant, and reacts with SO.sub.2 in the gas stream to produce Na.sub.2 SO.sub.4 which is collected in a baghouse or on plates of an ESP.
However, I have observed that the use of sodium can result in production of a reddish-brown plume coloration in stack gases downstream of the particulate control device. The use of the sodium bicarbonate not only removes SO.sub.2, but also removes some NO.sub.x (NO and NO.sub.2). While the precise mechanism is not known at this time, it is presently thought that some step within the overall sulfation reaction (reaction of sodium reagent with SO.sub.x) initiates the oxidation of NO to NO.sub.2. It is the presence of the NO.sub.2 in the exiting flue gases which is the source of the plume coloration.
Accordingly, this promising sodium reagent SO.sub.x control process has a serious disadvantage, in that its use creates an NO.sub.2 plume in the process of its removal of SO.sub.x pollution from flue gases. Accordingly, there is a great need for a method by which the NO.sub.2 produced by the use of sodium reagents can also be removed or suppressed, while not sacrificing SO.sub.x removal. This invention permits the injection of dry sodium reagents ahead of a baghouse without production of the plume.